In a heat pump, a refrigerant may flow in a cycle between two heat exchangers, typically coils. This cycle is called a vapor compression cycle. Heat pumps are often used to heat and cool a building or other structure. In such applications, one heat exchanger may be inside the structure (the “indoor heat exchanger” or “indoor coil”) and the other heat exchanger may be outside the structure (the “outdoor heat exchanger” or “outdoor coil”). For heating, the refrigerant may absorb heat as it passes through the outdoor heat exchanger and release heat as it passes through the indoor heat exchanger. For air conditioning, the refrigerant may absorb heat as it passes through the indoor heat exchanger and release heat as it passes through the outdoor heat exchanger. Heat pumps can reverse the direction of refrigerant flow, to change between heating and air conditioning. A reversing valve typically controls the direction of refrigerant flow.
Carbon dioxide (CO2) is a refrigerant with several desirable qualities. Carbon dioxide is inexpensive, abundant, and not flammable. Carbon dioxide also does not cause ozone depletion. However, carbon dioxide has a relatively low critical temperature of 87.7 degrees Fahrenheit. When used as a refrigerant in building heating and air conditioning, carbon dioxide frequently goes through “transcritical cycles,” flow cycles where the refrigerant exceeds critical pressure. Transcritical cycles are energy inefficient. Thus, carbon dioxide has not been commonly adopted as a refrigerant for building air conditioning and heating.
Certain devices, called cycle enhancements, can be inserted into a vapor compression cycle to improve energy efficiency. These cycle enhancements can partially compensate for some of the poor refrigerant characteristics of carbon dioxide. However, many cycle enhancements are one-way. One-way cycle enhancements function optimally only when refrigerant flows through them in a one direction. Conventionally, a reversible heat pump optimally benefits from one-way cycle enhancements in only one direction of refrigerant flow. One-way cycle enhancements may operate less efficiently, or may not operate at all, or may impede operation during the mode which has a reverse direction of refrigerant flow.
It would be desirable if a heat pump could fully benefit from one-way cycle enhancements regardless of the direction of refrigerant flow so as to benefit both the heating and cooling modes. Such a heat pump could lead to the adoption of carbon dioxide as a refrigerant in building air conditioning and heating.